International Concrete Abstracts Portal

Lattice girders consist of longitudinal reinforcing bars which are connected to vertical or inclined struts by welding. Due to the great stiffness of the anchorage of the struts, this kind of reinforcement works well as shear reinforcement also in two way span flat slabs. The experience with different kinds of girders as punching shear reinforcement led to an optimized girder shape. This highly effective girder was tested in full scale tests to obtain a European Technical Approval based on the European design code Eurocode 2. These tests are described in this paper. The results are evaluated according to the American design rules of ACI 318-14 too. The punching shear resistance of slabs with this special lattice punching shear reinforcement is compared with the resistance when using other reinforcement.

Inclined shear reinforcement affects the response to shear in reinforced and prestressed concrete members in two ways: it increases the capacity to resist shear and reduces the width of the shear cracks. The increase in shear resistance is the result of the decrease in the compressive stress in the diagonal compression struts – theoretically up to fifty percent - relative to beams with vertical stirrups. The reduction in crack width stems from the interception of the inclined shear cracks by the stirrups approximately at right angle. Associated with the reduced compressive stress in the concrete struts and the smaller crack widths is a reduction in shear deformation, which can be important for heavily loaded transfer girders or deep beams. For the understanding of the internal mechanism in concrete members with inclined shear reinforcement the paper first presents theoretical aspects for beams and relevant experimental verification from the literature, which confirm the above statements. This is followed by evidence from recent tests on punching shear in two-way slabs that the use of inclined headed stud shear reinforcement (HSSR) also increases the punching capacity of slabs.

The performance of slab-column connections has been critically studied over the last several decades by researchers aiming to better understand the behavior of flat slabs subjected to punching shear loading conditions. As a result, the use of slab shear reinforcement has emerged as a practical strategy to improve both the strength and ductility of reinforced concrete flat slabs.

The primary objective of this research study was to investigate the behavior of reinforced concrete slab-column connections employing an inclined shear reinforcement system comprised of deformed steel reinforcing bars. Results are presented from an experimental program conducted at the Ferguson Structural Engineering Laboratory of The University of Texas at Austin. The tests were aimed at establishing the merits and limitations of the shear reinforcement system, and it was found that a premature failure attributed to inadequate shear reinforcement anchorage controlled the performance of the strengthened slabs. The performance of the slabs constructed with the inclined reinforcement system is compared to that of slabs reinforced with more conventional, vertically-oriented, shear reinforcement. Lastly, the influence of the observed anchorage-driven failures were examined in the context of estimated slab shear resistances developed from provisions and analysis methods currently available for reinforced concrete flat slabs.

Current modeling procedures used to investigate the performance of reinforced concrete structures under impact are almost entirely confined to hydrocode approaches (e.g., LS-DYNA). While such procedures are capable of providing highly detailed representations of reinforced concrete structures and elements, they have often met with limited success due to the fact that: i) they typically employ complex micro-modeling representations of the structure under consideration, which can be expensive in preparation and computation, ii) they often require extensive characterization of material properties which are typically unknown, or calibration against previous test data, and iii) many of the commercial programs have shown deficiencies in their abilities to adequately capture cracked concrete response, particularly with regard to brittle shear-governed behavior.

This paper summarizes the application of an alternative modeling procedure for reinforced concrete slab and shell structures subjected to blast and impact loads. The nonlinear finite element program employed uses a layered thickshell element with reinforced concrete constitutive modeling done in accordance with the formulations of the Disturbed Stress Field Model (DSFM), a smeared rotating crack procedure shown to be capable of accurately capturing the behavior of shear-critical elements under conventional static loading conditions. This approach differs from that typically used within hydrocodes and results in comparatively simple model construction and reduced computation costs. The program is used to model the response of intermediate-scale reinforced concrete and steel fiber-reinforced concrete (SFRC) slab-like elements tested under repeated high-mass low-velocity impacts. Using simple finite element meshing techniques and predefined material behavioral models requiring only basic user input, good correlation between the observed and modeled slab response was attained.

The boundary conditions at the supports of reinforced concrete slabs, specifically the amount of lateral and rotational restraint, dictate how they respond to particular loads. Membrane (in-plane) forces are present in slabs when their boundaries are sufficiently stiff, therefore restricting the slabs from lateral translations in addition to rotations. Increases in compressive strength and ductility in ultra-high-performance concrete (UHPC) introduce additional strength enhancement not present in Normal-Strength Concrete (NSC).

Ten reinforced concrete slabs were quasi-statically tested in a static water chamber that allowed hydrostatic forces to be utilized as a loading technique on the slab. Of the 10 slabs, 4 were simply supported, and the remaining 6 were rigidly restrained. The load-deformation responses of laterally restrained slabs were then compared to those of simply-supported slabs to determine the enhancement due to the boundary conditions (i.e., compression membrane action). The results of these experiments were then compared to the results of response calculations based on plastic theory.

Valuable data on rigidly-restrained UHPC slab response were obtained from the experiments. The experimental results were compared to the results of the associated numerical analyses. Existing plastic theory should be used with caution when calculating the ultimate resistance of UHPC slabs. The experimental and numerical results showed that UHPC slabs with sufficiently rigid boundary conditions have a static resistance two-and-a-half-times greater than the traditional yield-line theory resistance for UHPC slabs due to compressive membrane effects.